There are many articles, such as semiconductors chips for example, which, during manufacture, become damaged and are either immediately rendered inoperative or have their operating lifetime reduced when their exposed surface(s) becomes scratched or stained. Thus, during the manufacture of such articles, one or more visual inspections are commonly performed to detect any surface defects, so that articles bearing a defect can be repaired, if possible, or if not, they can be scrapped prior to any subsequent processing. Detection of defects at an early stage of manufacture helps to reduce manufacturing costs and improve manufacturing yields.
Visual detection of surface defects is a relatively easy task when the article has relatively large surface features. Detection of defects becomes more difficult when the surface features are small, as in the case of a semiconductor chip having an exposed surface whose features have a linewidth often no larger than several microns. The small linewidth of the features on the exposed surface of the semiconductor chip has heretofore made it impractical to employ present day automated vision equipment to detect defects on the exposed chip surface. This is because most automated vision equipment accomplishes inspection by the technique of "pattern matching," which involves matching the image of the pattern of the article undergoing inspection to that of a perfect or "golden" pattern.
The process of detecting defects by the technique of pattern matching requires that the image of the pattern on the article undergoing inspection be accurately registered with the golden pattern. Otherwise, a match between the pattern on the article and the golden pattern becomes practically impossible to obtain. Registration of the pattern of the exposed surface on each of a plurality of chips, formed on a wafer, with a golden pattern, representing a set of perfectly formed chip features, is possible because one or more fiducials are usually present on the wafer. However, once the wafer is diced to separate the chips, it is difficult to register the pattern on each chip with the golden pattern because of the extremely small size of the features and the absence of any fiducials on the chip.
There is a need for a technique for visually detecting very small surface defects, such as scratches or stains, on a semiconductor chip, because often, such defects do not manifest themselves during electrical testing of the chip. As a result, when a semiconductor chip bearing a stain or crack is packaged to form an integrated circuit, there is the likelihood that the integrated circuit may prematurely fail in the field as a result of vibration and thermal cycling.